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11-17 of 17 messages
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RE: AL-1500 conductor heat problems
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by KB9WIS on February 17, 2006
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Tom,
Back to my original idea, the AL-1500 uses some doorknob capacitors in the tank, it looks like they are the 7.5kv type.. Couldn't I just change their capacitance values inversely, to the amount of capacitance that was added via the second tube, and then put the coils taps back to where they were from the factory? Wouldn't that bring the capacitance back to what it was, before the 2nd tube was added.., due to also reducing the fixed doorknob capatance in tank.. Wouldn't that also bring the tank back to its Q?
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RE: AL-1500 conductor heat problems
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by WB2WIK on February 17, 2006
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I didn't see Tom answer, so I don't know if he's returning here.
The doorknob caps are padders that are switched in on the lower bands. Their values can be reduced, of course. But the real problem lies in higher band operation, usually: The doorknob padders are *not* used there, so there's nothing to reduce or eliminate, and the minimum value of the plate TUNE capacitor is too high. I *think* that's why Tom was talking about using vacuum caps -- they can be made with extremely low values of minimum capacitance.
This is a major change, and vacuum variable capacitors aren't cheap. They're also not so easy to use because they usually require a lot of rotations for a small capacitance change.
WB2WIK/6
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RE: AL-1500 conductor heat problems
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by N4DEK on February 18, 2006
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Sounds like the guy took what was already a marginal amp at best and made a piece of junk out of it and then you bought it. I agree with WB2WIK, either convert it back to original or pawn it off on some other poor fool and buy a good amplifier.
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RE: AL-1500 conductor heat problems
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by W8JI on February 19, 2006
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I already answered the Q question, but let me reword it.
There isn't anything you can adjust or change on bands like 160, 15, or ten meters that allows a reduction of operating Q at any given power level. The exception would be to modify the tank to use a vacuum variable that have very low minimum C and higher maximum C, and change the tank coils and bandswitch.
You could change the coil tap positions on lower bands like 80, 40, and 20 and adjust for lower operating Q, but changing 15 and 10 would require an entirely new tank system including bandswitch and changing 160 woukld require a new LF tank and L coil.
73 Tom
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RE: AL-1500 conductor heat problems
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by KB9WIS on February 19, 2006
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Thanks everyone, I perhaps was a fool for even thinking this would of been a good amplifier, and trusted the Ham had everything, including tank/Q designed correctly..
Tom, the good thing about Ebay, is that used/surplus variable vacuum capacitors can be found cheaply.. From what I gather, I can simply "tune" the tank to a lower Q, via the use of a variable capacitor with a larger range of both "lower" and higher capacitance settings.., and then doing a little bit of retapping on the coil.. This really doesn't seem like too much of a chore or investment.., so I may give it a shot..
Let me know if I need to do any other changes/calculations, but I think the vacuum capacitor and new tappings on coil may be a simple procedure, once I get the right range of variable capacitor.. I imagine, I simply need to adjust the "tune" of new vacuum variable capacitor to a low capacitance setting, that would be about 20pf lower than the previous variable capacitor, as I believe the 2nd 8877 adds about another 20pf to the tank, and leave it there, and then move the tapping on coil, until back in resonance.. Hopefully the bandswitch will work, without changing it to another one, as they may cost a pretty penny.. Let me know if I'm getting on my facts right here..
Also:
I also wonder, that instead of using a different variable capacitor, if I could perhaps simply add a doorknob capacitor in "series" to the current variable capacitor to lower its capacitance on 10-20 meters, and then add some larger value doorknob capacitors in parallel to the current "tune" variable capacitor, to help bring 160 back into Q.. Would that also work?
Thanks again for your patience, and any thoughts/pointers would be appreciated,
kb9wis
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RE: AL-1500 conductor heat problems
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by W8JI on February 19, 2006
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I don't know why people want to do things like this. If the amp was left alone, it would run 2kW plus on SSB and CW for many years.
Using a vacuum cap will allow you to eliminate all the wiring and parts associated with bandswitching to 160 with the air variable.
As for two tubes, no one would want to do that without running 4kW out. The problem with this is you need 800 pf to have a Q of ten on 160, and it would be about 29pF on 10meters. This assumes you have a supply that is at 3500 volts full load of 1.8 amperes. This also assumes stray C is negligible outside of the tubes.
Circulating current in the inductor and bandswitch is over 18 amperes. This will never work with the bandswitch and tank coil size except very short duty cycle. You are talking big copper and a very wide contact switch.
With a single tube and 1500W you are back to 13 amperes tank current and a 370pF variable on 160, 15pF on ten. The switch and inductors will work OK at this level.
I'd convert back to one tube, or at least run a Q of ten at less than 2kW output with the existing switch.
I can't see any reason to run two 8877's. Even the big Alpha 77DX with the biggest transformer installed, you are lucky to make 3500 watts or so on a good meter wth two tubes. That's only a dB or two above a single tube.
73 Tom
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RE: AL-1500 conductor heat problems
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by KB9WIS on February 21, 2006
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Tom,
We you referring to the "Tune" or the "Loading" variable capacitor, that needed some of its capacitance range extended, to be lower, yet just as high for 160 meters? I really don't operate 160, so I may just need to lower one of them, via taking plates out, and/or adding a series capacitor..
I believe the Tune is the 4.5 KV variable capacitor, and the "Load" variable is about 1.5kv..
Just want to make sure.., before I try changing them out, and/or trying to add a doorknob capacitor in series trying to lower their capacitance..
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